How much does it reduce the risk is more debatable. It also reduces the risk of ovarian and possibly endometrial cancers as well. In f act pregnancy
itself may help reduce the risks of these also. Here are a couple of studies from reputable journals that demonstrated reduction in breast cancer
risk for women that breast fed:
if women who do not breastfeed or who breastfed for less than 3 months were to do so for 4 to 12 months, breast cancer among parous premenopausal women could be reduce by 11%; if all women with children
lactated for 24 months or longer, the incidence might be reduced by nearly 25%.
Source: newcomb, p. Etal. "lactation
and reduced risk of premenopausal breast cancer." n engl j med 1994; 330(2):81-87.
Women who were breastfed as infants, even if only for a short time, showed an approximate 25% lower risk of developing premenopausal or postmenopausal
breast cancer, compared to women who were bottle-fed as an infant.
Source: freudenheim, j. "exposure to breast milk in infancy and the risk of breast cancer." epidemiology 1994 5:324-331.